Carbon fibers are made from a number of precursor materials including primarily viscose rayon (regenerated cellulose), polyacrylonitrile (PAN) and pitch, and sometimes other precursor resins as well, attention being particularly invited to the background portion of co-pending application Ser. No. 07/534,075. A recent patent (U.S. Pat. No. 4,921,656) discloses the formation of a melt spun PAN precursor for carbon fibers.
Conventional carbon fibers are used in thermal insulation environments to replace asbestos for many purposes, such as furnace insulation, brakes including aircraft, automotive, truck, and off-road vehicle brakes, passive fire protection, etc. In brakes, carbon fibers are used in a carbon matrix to provide a carbon-carbon structure.
Concerning carbon-carbon materials, the Kirk-Othmer Encyclopedia of Chemical Technology (3rd Ed. 1980), Vol. 12, page 463 states:
Carbon represents the ultimate high temperature end-member of polymer matrix materials. It has one of the highest temperature capabilities under non-oxidizing conditions among known materials (it melts or sublimes, depending on the pressure, at 3550.degree. C.). Additional considerations of chemical and thermal compatibility make it natural to use carbon and graphite fibers as the reinforcement material. The resultant carbon-carbon . . . composites . . . are especially desirable where extreme temperatures may be encountered, such as in rocket nozzles, ablative materials for re-entry vehicles and disk brakes for aircraft. Other uses include bearing materials . . . and hot-press die components. PA0 One of the consequences of insufficient stabilization in a diffusion controlled stabilization process is the development of a hole in the center of such fibers during carbonization. The holes form as a result of the incompletely stabilized core of the precursor fibers being burned off during carbonization.
The specific gravity of carbon fibers depends on a number of factors including the nature of the precursor material and the degree of crystallinity (if any) in the resultant carbon fiber. Thus, well-ordered graphite molecular structure is dense. Novoloid precursor based carbon fibers are amorphous and have a relatively low specific gravity, whereas carbon fibers based on PAN are much denser having a normal specific gravity (g/cm.sup.3) of 1.8-2.0. The Kirk-Othmer Encyclopedia of Chemical Technology (3rd Ed 1981) , Vol 16, page 135 contains a table (Table 3) showing typical properties of carbon fibers.
The most commonly used PAN based carbon filaments have a specific gravity of 1.75 (e.g. Hercules AS-4), whereas the most commonly used pitch based carbon filaments, mesophase pitch based, have a specific gravity of 1.85 to 2.10.
In structural applications, there is an important relationship between the weight of the fiber and its strength. Carbon fibers are often used in place of glass fibers as reinforcement in order to save weight, for example in aircraft and space structure where weight is critical. In aircraft, carbon fiber is used for reinforcement of primary and secondary structures and interior parts such as flooring, luggage bins, ducting, etc. While conventional carbon fibers are very useful in the environments noted above and have an excellent strength to weight ratio, the need exists for fibrous reinforcing materials having an even better strength to weight ratio.
Carbon fibers are also used in a variety of miscellaneous environments such as for high temperature gaskets, seals, pump packing, medical implants, cement reinforcement, etc. Most fiber-reinforced plastics are laminated materials. The fibers in each layer are usually arranged in one of four configurations such as in the form of uni-directional tape, woven fabric, chopped and aligned fibers or randomly disposed fibers in the form of a mat or non-woven fabric.
Another problem with conventional carbon fibers is that they have a tendency to be non-circular, i.e. either during spinning of the precursor or transformation of the precursor fiber to carbon fiber, the cross-section tends to flatten or distort into a more or less dog-bone or kidney-shaped cross-section, and this problem especially occurs in the manufacture of conventional dry spun carbon precursors from PAN, and from rayon precursor. Carbon fibers which are somewhat flattened or kidney shaped in cross-section provide somewhat irregular and more difficult to control properties in the final article.
A further problem for the PAN precursor carbon fiber is that a shrinkage of approximately 40-50% occurs in converting the precursor into the carbon fiber. As a result, longitudinal surface cracks tend to occur which reduce the physical properties of the resultant carbon fiber.
In a conference held on Apr. 25, 1990, Dr. A. S. Abhiraman, Georgia Institute of Technology, made a presentation concerning the accidental production of carbon fibers with holes. A slide was shown of a cross-section of such a fiber, which cross-section was of a doughnut shape; on the other hand, these fibers were not characterized as being hollow fibers and the inference drawn from the materials presented suggested that the holes produced were non-continuous. Dr. Abhiraman indicated that these fibers were produced by accident by faster than normal rate of carbonization and indicated that he had found no uses for such carbon fibers with holes and that they are of no known value. The physical properties were not discussed and no conclusions can be drawn as to the brittleness, tensile strength and other properties of carbon fibers with holes as so produced by a faster than normal rate of carbonization.
Dr. Abhiraman and his colleagues have a number of publications. Grove et al, in Carbon, 26(3), 403-11 (Eng 1988) in an article entitled "Exploratory Experiments in the Conversion of Plasticized Melt Spun PAN-Based Precursors to Carbon Fibers" discusses exploratory experiments in the conversion of plasticized melt spun PAN-based precursors to carbon fibers wherein, in some cases (see the abstract), "the precursor fibers have broken filaments as well as surface defects and internal voids, all of which hinder the development of superior properties". In the concluding remarks at pages 410 and 411 it is indicated, "Microholes were observed frequently in SEM examination of cross-sections of . . . fibers, suggesting the presence of impurities in the precursor fibers. Surface flaws and impurities will have to be reduced for these melt spun, PAN-based precursor fibers to become a viable alternative to current wet or dry spun, acrylic precursors".
Balasubramanian et al (Bienn. Conf. Carbon, 17th, 312-13, Eng. 1985), in an article entitled "Evolution Structure and Properties in Continuous Carbon Fiber Formation" discusses certain PAN based carbon fibers, some of which are said to have hollow cores (see Table 2). Column 2 on page 312 indicates that a hollow core forms when the fibers are incompletely stabilized and that it is necessary to properly stabilize to avoid the formation of a hollow core in the carbon fibers, the formation of the hollow core being caused by burning-off of an incompletely stabilized core as well as the rapid development of a rigid skin. It appears quite clear that the formation of a hollow core is undesirable.
Abhiraman, in "From PAN-Based Precursor Polymers to Carbon Fibers: Evolution of Structure and Properties" appearing in Adb. Mater. Technol. '87, 945-52 [Eng] 1987) mentions the production of hollow core carbon fibers and indicates what parameters must be controlled "in order to avoid core blow-out in carbonization". Again, there appears to be no disclosure of carbon fibers which are continuously and generally uniformly hollow, no disclosure that any such carbon fiber might be useful, and no disclosure of the manufacture of a hollow carbon fiber from a hollow precursor.
Balasubramanian et al, in "Conversion of Acrylonitrile-Based Precursor Carbon Fibers--Part 3--Thermo-oxidative Stabilization and Continuous, Low Temperature Carbonization" appearing in Journal of Material Science 22 (1987) 3864-3872, mention hollow core carbon fibers. The following statement appears in the last paragraph on page 3866:
It is evident that the irregularities produced, including hollow core, are to be avoided. There is no indication of such fibers being useful, no indication of the production of hollow fibers which have a generally continuous and uniformly hollow core, and no disclosure of the manufacture of hollow carbon fibers from hollow precursor fibers.
In a "Materials and Processing Report" dated May, 1986 from Massachusetts Institute of Technology, Volume 1, No. 2, it is indicated that the researchers Dan Edie and Charles Fain of Clemson University found that they could increase the tensile strength of melt-spun pitch-based carbon fibers by modifying the flow profile during extrusion to form both non-circular and hollow fibers. In an undated report in the names of Harrison, Fain and Edie (Clemson University) entitled "Study of Hollow and C-Shaped Pitch-Based Carbon Fibers" it is seen that the hollow carbon fibers in question were of very large size, these having a typical outside diameter of 40-50 microns and a wall thickness of 8-15 microns providing a fiber cross-sectional area of up to 1000.mu..sup.2 in comparison with a typical 11.3 micron carbon fiber having a cross-sectional area of only 100.mu..sup.2 (page 79 of the undated report). Also see "Melt-Spun Non-Circular Carbon Fibers" by Edie, Fox, Barnett and Fain appearing in Carbon, Volume 24, No. 4, pp. 477-482 (1986).